1. Field of the Invention
The present invention relates to a method and a system for enhanced ratio control in a toroidal drive.
2. Description of the Background Art
Continuously variable transmissions (CVT""s) are transmissions that change a speed ratio continuously, not in discrete intervals. This continuous nature of CVT""s gives them an infinite number of speed ratios, making them very attractive for automotive use.
Various types of CVT are known. One such example is a CVT with pulley/V-belt power transfer. Another example is a CVT with disc/roller power transfer. The CVT of this type is often referred to as a toroidal-type CVT (T-CVT) because it transmits torque from one rotating semi-toroidal disc to another semi-toroidal disc by traction rollers through a traction force. The two semi-toroidal discs form a toroidal cavity. In each toroidal cavity, it is preferred to have two traction rollers in equiangularly spaced relationship engaging the discs for transmission of motion therebetween. While three or four traction rollers may be disposed in spaced relationship in each toroidal cavity and will provide increased life for contact surfaces as the total surface area is increased, two traction rollers are preferred for simplicity.
Each traction roller is rotatably supported by a pivot trunnion, respectively. The pivot trunnions, in turn, are supported to pivot about their respective pivot axis. In order to controllably pivot the pivot trunnions for a ratio change, a hydraulic control means is provided. The hydraulic control means is included in a hydraulic cylinder at each pivot trunnion and includes a control volume defined in the hydraulic cylinder between a piston and an axial end of the hydraulic cylinder The pistons within the hydraulic cylinders are connected to the pivot trunnions along their pivot axis by rods. The piston and its associated rod are thereby rotatable about the pivot axis with the associated pivot trunnion. Variation of the control volume causes the piston to move relative to the hydraulic cylinder, and applies a control force to displace the pivot trunnions. Control forces applied displace the pivot trunnions in the opposite directions along their pivot axis. As a result, the pivot trunnions are caused to pivot about their respective pivot axis, due to the forces present in the rotating toroidal discs, for initiating ratio change.
For terminating the ratio change when a desired ratio has been obtained, a feedback structure is provided. The feedback structure preferably includes a source of hydraulic pressure, and a ratio control valve for controlling the flow of hydraulic fluid for initiating ratio change. The feedback structure further includes a mechanism associated with at least one pivot trunnion to adjust the ratio control valve upon pivotal movement of the pivot trunnion to a desired ratio. The mechanism is preferably a cam connected to a pivot trunnion. The cam may be linked mechanically and/or electronically to operate the ratio control valve upon reaching a desired rotation.
Various ratio control strategies have been proposed. One such example is proposed by the assignee of the present invention in U.S. Pat. No. 5,669,845 (=JP-A 8-270772) issued Sep. 23, 1997 to Muramoto et al. According to this known control strategy, a feedback structure includes a source of hydraulic pressure, a ratio control valve, a bell crank, and a cam. The ratio control valve has a valve sleeve connected to a stepper motor. The ratio control valve further has a valve spool disposed within the valve sleeve. The valve spool has a rod projecting out of the valve sleeve for engagement with the bell crank. The bell crank is connected to the rod at one end. At the other end, the bell crank engages the cam connected to a pivot trunnion. At a middle point between the two ends, the bell crank is supported to pivot about the middle point.
The valve sleeve is positionable in response to an actuator command from a T-CVT controller to establish various speed ratios between input and output shafts of the T-CVT. The actuator command is indicative of motor steps of the stepper motor. The axial displacement of the valve sleeve has one-to-one and onto any selected number of motor steps.
To compute the number of motor steps, the T-CVT controller determines a desired engine or input shaft speed against vehicle speed and throttle position using a look-up table map. The desired input shaft speed is used in cooperation with actual output shaft speed to determine a desired ratio. Using a predetermined relationship, the T-CVT controller determines a desired trunnion angular position. Using the desired trunnion angular position, the T-CVT controller computes a feedforward term and a feedback term by carrying out proportional and integral control actions. Besides, the T-CVT controller computes a damping term using an estimated value of trunnion axial displacement given by a state observer. Combining the feedforward, feedback and damping terms gives the motor steps.
This known ratio control is satisfactory to some extent. As far as the inventors are aware of, huge amount of computer simulation and field test would be needed in designing such a T-CVT controller to ensure quick reduction of error in estimation, if occurred, by state observer, requiring increased cost and time in developing a desired control system.
Accordingly, a need remains for enhanced ratio control in a toroidal drive of a T-CVT, which does not require increased cost and time in developing a desired control system.
An object of the present invention is to provide a method and a system for enhanced ratio control in a toroidal drive of a T-CVT to meet the above-mentioned need.
According to one aspect of the present invention, a method for enhanced ratio control in a toroidal drive of a toroidal-type continuously variable transmission (T-CVT) is provided. The T-CVT includes a ratio control element positionable in response to an actuator command to establish various ratios between input and output shaft speeds of the T-CVT. The toroidal drive has toroidal discs defining a toroidal cavity, and traction roller assemblies having pivot trunnions rotatably supporting traction rollers disposed in the toroidal cavity and engaged between the toroidal discs. The method comprises:
computing a factor of proportionality by which a first physical quantity and a second physical quantity are related,
the first physical quantity being a trunnion axial displacement of a predetermined one of the pivot trunnions, the second physical quantity being indicative of a ratio rate of the ratio between the input and output shaft speeds of the T-CVT;
establishing a filter in the form of a characteristic equation that includes a third physical quantity and a fourth physical quantity, as inputs, a quasi-state quantity, as a state quantity, and coefficients including a transition coefficient for the quasi-state quantity, the transition coefficient including an observer gain,
the third physical quantity being indicative of the ratio between the input and output shaft speeds of the T-CVT, the fourth physical quantity being indicated by the actuator command;
computing the quasi-state quantity using the filter;
computing an estimated quantity of a system state quantity of the T-CVT using the quasi-state quantity, the observer gain, and a fifth physical quantity indicative of a trunnion angular position of the predetermined pivot trunnion, the system state quantity including at least the first physical quantity; and
correcting the observer gain in response to the factor of proportionality to keep the transition coefficient unaltered.